The present invention relates, in a general manner, to systems for loading and/or unloading of fluids, especially from vessels for transporting the said fluids. A preferred field of application is the transfer of liquefied natural gas between a floating production storage and offloading (FPSO) platform and an oil tanker moored near this platform.
Among the methods of exploitation of offshore oil fields, the use of these independent floating production platforms is expanding rapidly. The installations are moved successively onto the separate offshore deposits, which become economically viable once their exploitation no longer requires the installation of a permanently fixed infrastructure.
One of the key points in the chain of exploitation is the transfer of the products obtained from the FPSO to the vessel that is to transport them. This operation is carried out on the open sea and therefore is strongly dependent on the sea conditions.
For this purpose, there is already a proposal to provide the FPSO with loading arms similar to those used on wharfs, an example of which is described in document GB-2 042 466. To carry out the loading/unloading operation, the vessel and the FPSO must be moored side by side, in the same way as in a port with a jetty. However, this mooring side-by-side is only possible in a very calm sea.
The use of loading and/or unloading systems like those described in documents FR-2 469 367 and EP-0 020 267 has also been proposed. These systems include a device for transferring fluid between a loading jib mounted on the FPSO and a coupling means provided on the vessel. The transfer device comprises a system of multiple articulated segments for fluid pipe of concertina or deformable diamond-shape(s) type and actutated by cable, the ends of the network being connected, by means of bends and rotary joints, respectively to pipe sections fixed to the jib and pipe sections that are to be connected to the coupling means.
Such a system permits loading or unloading in tandem in rough seas. However, it takes up a lot of space on the FPSO.
Other systems propose the use of floating or suspended hoses between the FPSO and the vessel, which are moored side-by-side or in tandem.
Although these systems make it possible to effect loading in very rough seas, the loading rate is limited by the flow rate in the hoses. Furthermore, these hoses offer limited resistance to pressure surges and the large radius of curvature of the hoses means that a large storage volume is required (large-diameter drum). This type of hose also has a limited service life and requires restricting periodical tests. Above all, however, the present state of hose technology does not permit cryogenic transfer.
In other embodiments, hoses joined by rotary joints form product lines that are supported by an articulated metal structure.
The present invention aims to improve the conditions of transfer of fluid between two locations, in particular between a first location on a floating production storage and offloading platform and a second location on a vessel that is to transport the fluid.
For this purpose, it proposes an arrangement for transferring fluid between a first location and a second location, comprising:
a control winch under constant tension that is to be installed at the first location, on which a suspension cable is wound which is to be stretched between the two locations and which is able to subject the suspension cable to a constant tension;
a storage stand that is to be installed at the first location for storing suspended rigid pipe elements that are articulated together by means of articulation sections provided with bends and rotary joints, in a manner that makes it possible to pass from a storage position in which the pipe sections are suspended in concertina fashion on the storage stand to a position spread out between the two locations by suspension from the cable for carrying out the transfer of fluid; and
means for coupling certain predetermined articulation sections to the storage stand or to the suspension cable depending on the length of suspension cable stretched between the two locations.
Such an arrangement with rigid pipework, with the individual elements connected together with rotary joints, permits a high fluid velocity and hence a high transfer rate. It also gives the pipework good resistance to pressure surges.
In addition, it makes it possible to transfer liquefied natural gas using existing cryogenic rotary joints, such as Chicksan(copyright) rotary joints.
Furthermore, as the suspension cable is subjected to a constant tension, it is wound onto its winch or unwound from the latter as a function of the movement of mutual separation or approach of the two structures. The number of predetermined articulation sections hung on this suspension cable therefore depends on the length of the latter stretched between the two structures.
Preferably, the coupling means comprise a plurality number of struts for suspending the predetermined articulation sections, to each of which a collet is fixed transversely for holding the suspension cable from above, to fix the suspension strut to the suspension cable, and the arrangement includes in addition a connecting winch that is to be installed at the second location, on which a connecting cable is wound, and this is to be connected to the suspension cable for taking it, prior to transfer of fluid, to the second location and securing it there or for bringing it back, after transfer of fluid, to the first location, all the while subjecting it to a constant tension by means of the constant-tension control winch.
On account of these arrangements, the connecting winch extracts the suspension cable and the articulated pipe sections from the storage stand, whereas the constant tension of the constant-tension control winch resists the exit of this cable and limits the deflection or sag of the suspended assembly.
For taking the connecting cable to the first location and connecting it to the suspension cable, the arrangement includes, advantageously, a winch that is to be installed at the first location, and on which a rope is wound, which is to be joined to the connecting cable for taking it to the first location in order to connect it to the suspension cable.
For fixing the connecting cable to the suspension cable, a mechanism with clamps, capable of firmly joining one end of the connecting cable to the suspension cable, is preferably fixed to one end of the latter.
Again preferably, the arrangement includes a device forming a mechanical stop, which is to be installed at the second location and has the purpose of locking the clamping mechanism, once the suspension cable is stretched between the two locations.
For reasons of convenience, the arrangement includes a means of fluid connection on an end pipe section and it is intended to be connected to a complementary means of fluid connection that is to be installed at the second location for executing the transfer of fluid.
According to characteristics that are preferred from the standpoint of the possibilities of movement offered by the latter:
at least some of the articulation sections that are to be hung from the suspension cable have a combination of a rotary joint with approximately vertical axis and of at least one rotary joint with approximately horizontal axis, with the pipe sections in the spread-out position; and/or
the coupling means have a plurality of suspension struts, each of which has a collet for holding the suspension cable from above, fixed transversely to one of its ends, and is joined to an articulation section by means of a pivot whose axis is roughly parallel to the direction of extension of the channel for receiving the suspension cable defined by the collet; and/or
the coupling means have a plurality of suspension struts, each of which is joined to the articulation section by means of a rolling bearing.
According to a preferred embodiment, the storage stand is mounted freely pivoting in azimuth on a base that is to be fixed at the first location and the arrangement includes in addition at least two sets of pulleys for lateral guidance of the suspension cable, fixed to the storage stand in different locations and capable of moving away from the suspension cable alternately on passage of a coupling means.
Due to these arrangements, the storage stand is aligned automatically on the suspension cable, while offering lateral flexibility of the product line formed by the pipe sections.
According to one embodiment variant, the storage stand is mounted pivoting in azimuth on a base that is to be fixed at the first location and the arrangement includes in addition a detector of the angular position of the suspension cable and a device for rotational control of the storage stand about the base, which is sensitive to filtered output signals of the detector for aligning the storage stand in the principal direction of the suspension cable.
According to another variant, the storage stand is connected rigidly to a base that is to be fixed to the first location, each articulation section that is to be hung on the suspension cable has a combination of a rotary joint with approximately vertical axis and of at least one rotary joint with approximately horizontal axis, with the pipe sections in the spread-out position; and the assembly has at least two sets of pulleys for lateral guidance of the suspension cable, fixed to the storage stand in two different locations and capable of moving away from the suspension cable alternately on passage of a coupling means.
According to preferred characteristics for their convenience of implementation, the coupling means have a plurality of suspension struts, to each of which a collet is fixed transversely for clamping the suspension cable from above, each of the collets having two articulated arms, which are moved towards a clamping position of the collet by the action of a spring, and each one provided with a roller, and the stand having two rails, each defining a rolling track for one of the rollers of the collet, the spacing of the rails being such that in the position of storage of the pipe sections, the collet is maintained in an open position against the force of the spring, permitting engagement of the latter on the suspension cable during passage of the pipe sections to the spread-out position.
For supporting the suspension cable as it leaves the storage stand, the arrangement includes, advantageously, suspension cable supporting pulleys, downstream from the rails of the storage stand.
The present invention also proposes the use of the arrangement described above for the transfer of liquefied natural gas between a floating production storage and offloading platform representing the first location and a vessel representing the second location, the pipe sections being connected by articulations to other pipe sections to form two pipelines for transfer of fluid which can be deployed simultaneously and parallel between the two locations, one of these pipelines serving for transfer of liquefied natural gas to the vessel and the other serving for return, of the vapour to the platform.